1. Field of the Invention
This invention relates to a slip control system for a vehicle, and more particularly to a slip control system for a vehicle in which the accelerating performance when the slip of the control wheels is converging on a target value is improved.
2. Description of the Prior Art
There has been put into practice a traction control system which detects the amount of slip of the driving wheels and controls the engine output and/or application of braking force to the wheels so that the amount of slip of the driving wheels converges on a target value, thereby preventing deterioration of acceleration of the vehicle due to an excessive driving torque transmitted to the driving wheels when the vehicle is to be accelerated. Many vehicles are provided with a traction control system as well as an anti-skid brake system. See, for instance, Japanese Unexamined Patent Publication No. 1(1989)-197160.
In the traction control system disclosed in Japanese Unexamined Patent Publication No. 2(1990)-38149, the target value for the traction control on which the amount of slip of the driving wheels is to be converged is set smaller when the vehicle is making a turn.
In the conventional system, the lateral acceleration acting on the vehicle is detected and the target value for the traction control is set smaller as the lateral acceleration increases, thereby suppressing slip of the inner driving wheels and preventing deterioration of the driving stability.
Though the target value is set smaller as the lateral acceleration increases and the engine output is greatly suppressed when the vehicle is making a turn, the traction control is terminated and the normal control is resumed when the slip of the driving wheels converges on the target value for a predetermined time.
In the conventional traction control, the control variable is kept as it is to keep the engine output suppressed for said predetermined time even after the slip of the driving wheels converges, which results in deterioration of the accelerating performance immediately before and after the end of the traction control.
When the lateral acceleration is large, the target value for the traction control is set smaller, when the friction coefficient .mu. of the road surface is high, the target value is set smaller and when the vehicle is making a sharp turn and the amount of slip of the inner driving wheel is large, the control variable of the traction control is set larger, and accordingly the accelerating performance immediately before and after the end of the traction control is especially deteriorated in these conditions.
Further when the friction coefficient .mu. of the road surface suddenly changes from low to high, slip of the driving wheels quickly converges. Also in such a case, the accelerating performance can deteriorate.